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ON THE CHANGE  OF REFRANGIBIHTY  OF LIGHT.          281
I
compound character of the dispersed light, notwithstanding the perfect homogeneity of the incident light.
18. The third method of observation is well adapted to bring into view the variation in the absorbing energy of the medium corresponding to a variation in the refrangibility of the incident rays. When the eye is placed vertically over the vessel containing the solution, so that the dark planes corresponding to the fixed lines of the spectrum are projected into dark lines, of which                       |
the length is not exaggerated by obliquity, the boundary of the                       I
dispersed light is projected into a curve, which serves to represent                       (
to  the  eye   the  relation between  the  absorbing power of the                      "
medium and the refrangibility of the incident light. This curve is not exactly that which Sir John Herschel has treated of in                      I
the theory of absorption, and considered as the type of the absorbing medium to which it is applied, but nevertheless it                      -;
serves much the same purpose. It is true, that, independently of any change in the absorbing energy of the medium^ an in-                      4;
creasing faintness in the dispersed light would produce to a certain extent an approximation of the curve to its axis; but practically, in the case of sulphate of quinine, as well as in a                       >
great many others, the appearance is such as to leave no doubt as to the existence of a most intense absorbing energy on the                       t,f
part of the medium with respect to rays of very high re-frangibilities*.
In the case of a solution of sulphate of quinine of the strength of one part of the disulphate to 200 parts of acidulated water, it has been already stated that a portion of the rays which are capable of producing dispersed light passed across a thickness of 3 inches. On forming a pure spectrum, the fixed line PI was traced about an inch into the fluid. On passing from PI towards <?, the distance that the incident rays penetrated into the fluid increased with great rapidity, while on passing in the contrary direction it diminished no less rapidly, so that from a point situated at no great distance beyond H to where the light ceased, the dispersion was confined to the immediate
* I should here remark, that, after the researches described in this paper had far advanced, I met accidentally with a passage in the Comptes Eendus, torn. xvn. p. 883, in which M. Ed. Becquerel mentions a solution of acid sulphate of quinine as a medium eminently remarkable for its absorbing power with respect to the rays more refrangible than H.